The present invention relates to bearings, and more specifically, to surface strengthening techniques for bearing components.
Inclusions and porosity in metals are detrimental to the performance of highly stressed mechanical components, such as bearing components (e.g., bearing raceways). In the case of powder metallurgy, powder metal (“PM”) components inherently include porosity that results in reduced strength, making them unsuitable for various highly stressed applications. The strength of PM materials increases with a reduction in porosity. Techniques such as double-press, double-sinter, powder forging, and others have been used to reduce porosity and improve the strength of PM components. Additionally, selective densification at and near the surface of components improves the rolling and sliding contact fatigue behavior of compacted and sintered materials.
Forming mechanical components using a powder metallurgy process has many advantages, such as being able to produce parts with complex geometry near final net shape with very little or no machining operations. The typical powder metallurgy manufacturing process typically includes compacting a selected powder mix under high pressure into a shape known as a pre-form. The pre-form is then thermally treated by a process known as sintering, which causes the powder particles to fuse together. The strength of the PM part is directly related to its density. Density of pressed and sintered products depends upon the pressure at which they are compacted. Because compaction pressure is limited by the strength of the compaction tooling, sometimes multiple pressing operations (e.g., double-press) are conducted on the sintered part to increase its density. To achieve 100% density, the sintered PM part is further hot forged. To perform all these operations significantly increases the cost of manufacturing, which makes PM unattractive in the case of bearing components.
As briefly mentioned above, the surface of less than 100% densified components may be selectively strengthened via densification by the application of mechanical pressure. This can be achieved by, for example, rolling a hard roller over the surface (i.e., burnishing) and/or localized hammering (i.e., peening). Burnishing and peening help extend the operational life of the components under cyclic fatigue conditions. Previously, these processes were usually only able to accomplish densification to a depth of less than 0.5 mm, with some processes able to densify only up to 1 mm below the surface. Also, some of the pores may not be effectively closed with typical burnishing and peening techniques, which results in lower performance under rolling contact fatigue conditions.